The power of a laser is often measured on line by sampling the laser beam during the operation of the laser and calculating the power of the transmitted, or usable, beam based on the power in the sample. One method of sampling the beam on line is to place a beamsplitter in the path of the beam after the beam has exited the laser cavity, that is, beyond the front mirror, and "split off" a portion of the beam for sampling. The power in the split-off portion of the beam is measured using any of a number of optical measuring devices, and the power of the transmitted beam is then calculated. Sampling the laser beam with a beamsplitter has at least two disadvantages--the usable power of the laser beam is reduced by the power which is split off, and the spatial profile of the transmitted beam may be distorted by the beamsplitter.
Another method of sampling a laser beam on line is to use a rear mirror which is designed to permit a small amount of the beam to "leak" through the mirror. The leaked power is then measured and the power of the transmitted beam is calculated. This method of sampling the laser beam on line has the same disadvantage of reducing the usable power of the laser, and a further disadvantage of inaccuracy because it samples the beam at the rear mirror, before the beam has been fully amplified in the laser cavity, rather than sampling the beam as it exits the laser cavity through the front mirror. In addition, slight changes in reflectivity of the rear mirror may introduce large errors in the measured power.